Avast repeatedly asking for boot time scan

As above… So I say OK. It reboots, runs a scan and then says ‘Some viruses are too difficult,’ so I should boot from a rescue disk. I do that and it comes up clean, than an hour later it asks for a boot time scan again

Malware removal assist is done in this forum section

viruses and worms >> https://forum.avast.com/index.php?board=4.0